1. Field of the Invention
The present invention relates to an image scanner in which a scanning speed is controlled. The image scanner is used in an image forming apparatus, such as a copy machine, a printer and a facsimile.
2. Discussion of the Background
Currently, there exists image scanners in which a scanning speed may be controlled. In these image scanners, a scanning operation is executed by using a stepping motor and the scanning speed is controlled by controlling a pulse rate for the stepping motor.
The image scanners include a Central Processing Unit (CPU) which calculates the pulse rate for the stepping motor according to a system clock. The system clock is divided and input to the CPU. The CPU then controls the stepping motor using an external port.
In a scanner, a scanning speed is changed for scaling a sub-scanning direction. For this changing scanning speed, a value of an internal timer of the CPU is changed to change the rotation speed of the stepping motor.
Most scanners include a scaling function which allow, for example, a magnification of 25%-400% to be set in increments of 1%. However, smaller steps, for example 0.1% are desired.
For the case of a 25%-400% magnification in steps of 1%, a minimum resolution of 0.3%-0.5% is achieved. The minimum resolution is decided according to characteristics of a motor, a lamp for illumination, mirrors, and a mass of a moving device used to move the mirrors at a desired scanning speed.
In this case, a ratio of the scanning speed between the minimum magnification 25% and the maximum magnification 400% is 16 (i.e., =400/25). Therefore, the scanning speed of the minimum magnification is 16 times the scanning speed of the maximum magnification.
Further, in a scanner which has a carriage used for moving an optical unit, when a scanning operation is completed, the carriage is returned to a home position. Therefore, the rotating speed of the motor is decided according to a speed of the carriage returning as well as a power of the motor.
When a count number of the CPU for rotating one angular step (for example, 0.72.degree.) is 340, a minimum resolution is about 0.29% (=1/340). Therefore, a count number of more than 1000 is needed for rotating one angular step for setting the magnification by a 0.1% step. That is, when a count number of the CPU for rotating one angular step is 1000, a minimum resolution is 0.1% (=1/1000).
To execute the 1000 counts, it is necessary to increase a drive ratio of the stepping motor or to increase the number of base clocks of the CPU. However, both of these ways have problems. In the case of increasing the drive ratio of the stepping motor, a moving speed of a carriage becomes deficient. Further, because a CPU has a limited number of base clocks, it is difficult to increase the number of of base clocks the CPU to execute 1000 counts.